1. Field of the Disclosure
This specification relates to a safety system for securing safety of a nuclear power plant, and in particular, to a facility that may decrease the concentration of a radioactive material in a containment by a passive principle when an accident occurs in the nuclear power plant and a nuclear power plant having the same.
2. Background of the Disclosure
Depending on the position of installation, nuclear reactors are classified into loop-type reactors (e.g., commercial reactors, Korea) with main components (steam generators, a pressurizer, reactor coolant pumps, etc.) installed outside the reactor vessel and integral reactors (e.g., SMART reactor, Korea) with the main components installed in the reactor vessel.
Further, nuclear reactors are classified into active reactors and passive reactors depending on how the safety system is implemented. The active reactors are reactors that use an active component, such as a pump, which is powered by an emergency diesel generator in order to operate the safety system, and the passive reactors are reactors that use a passive component which is powered by a passive force such as gravity or gas pressure in order to operate the safety system. In the passive reactors, the passive safety system may safely maintain the reactors only with a natural force embedded in the system without a safety-grade AC power source such as an emergency diesel generator or an operator's action at least for a time (72 hours) required by the regulations when an accident occurs, and after 72 hours, the passive safety system may be treated by the operator or assisted by a non-safety system.
A containment (containment building, reactor building, containment vessel or safeguard vessel) that plays a role as a final protection barrier to prevent radioactive materials from releasing to the external environment are classified into the containment building (or reactor building) formed of reinforced concrete and the containment vessel and safeguard vessel formed of steel depending on the material constituting a pressure boundary. The containment vessel is a large vessel that is designed to have a low pressure like the containment building, and the safeguard vessel is a small vessel designed to be rendered to have a small size and having the higher design pressure. Unless mentioned specially, as used herein, the terms “containment building,” “reactor building,” “containment vessel,” or “safeguard vessel” are collectively referred to as a containment.
Various forms of active and passive systems, such as a containment spray system, a containment cooling system, a suppression tank or suppression pool, are put to use in order to decrease the density of radioactive material, the pressure and temperature in the containment at accidents. Hereinafter, such facilities are described below one by one.
The active containment spray system (Korean commercial reactor, SMART reactor, etc.) sprays a large amount of cooling water using containment spray pumps when an accident occurs, recollects the cooling water to an in-containment refueling water storage tank or sump, and re-sprays the cooling water to decrease the pressure and temperature of the containment and the concentration of radioactive material for a long time. The active containment spray system may perform a long-term spraying function and requires a power system to be available for activating the pumps.
The passive containment spray system (Canadian CANDU, etc.) has a cooling water storage tank at an upper side of the containment and sprays a large amount of cooling water when an accident takes place to decrease the pressure and temperature inside the containment and the concentration of the radioactive material. Since the passive containment spray system has a limited storage capacity of cooling water, and thus, cannot be operated more than a predetermined time. Accordingly, the cooling water storage tank needs to be periodically made up using a pump for long-term use of the passive containment spray system. This means that the passive containment spray system also needs to use a pump and a power system for activating the pump in order for a long-term operation.
The suppression tank (commercial BWR, CAREM: Argentina, IRIS: Westinghouse, U.S. et. al.) guides the steam discharged into the containment to the suppression tank using a difference in pressure between the containment and the inside of the suppression tank and condenses the steam to decrease the pressure and temperature in the containment and the concentration of the radioactive material. The suppression tank operates only when the pressure in the containment is higher than the pressure in the suppression tank.
The passive containment cooling system has heat exchangers and a cooling water tank installed in or outside the containment and condenses the steam in the containment using the heat exchangers to decrease the pressure and temperature in the containment and the concentration of the radioactive material. The passive containment cooling system uses the natural circulation in the containment and thus has a lower performance in reducing the pressure and temperature and concentration of radioactive material as compared with the active containment spray system.
Besides, there is a sort of passive containment cooling system (AP1000: Westinghouse, U.S.) that applies a steel containment vessel to cool (spray, air) the external wall and that condenses the steam in the containment vessel on the internal wall of the containment vessel to thus decrease the pressure and temperature in the containment vessel and the concentration of radioactive material. This system uses the natural circulation in the containment similarly to the passive containment cooling system and thus shows a relatively low performance in reducing pressure and temperature and the concentration of radioactive material as compared with the active containment spray system.
Most of the above-described systems show a relatively excellent performance in decreasing the pressure and temperature inside the containment. However, among the radioactive materials that may spread to the external environment when an accident occurs in the nuclear power plant, iodine may have a highest proportion of concentration. Iodine, when contacts water, is mostly dissolved in the water (solubility 0.029 g/100 g(20° C.)). Accordingly, among the containment-related safety systems, the active containment spray system (which is adopted for the Korean commercial reactors), which uses an active pump to spray a great amount of cooling water and to recirculate the cooling water for a long time, may show the most excellent performance in decreasing the concentration of radioactive material in the containment. However, the active safety system necessarily requires supply of emergency AC power for operating the active components such as pumps when an accident occurs in the nuclear power plant, and without supply of emergency AC power, does not operate.
In this point of view, demand for the passive safety system with relatively high safety is on the rise. This is why the passive safety system does not require a power system nor continuous operation of the active components. However, in case the passive safety system is adopted as safety system of the containment, the concentration of radioactive material in the containment would be relatively higher due to a lower performance in containment cooling as compared with the active safety system.
Further, an exclusion area boundary (EAB) is set for the public safety to restrict the public access in preparation for an accident that may occur in the nuclear power plant. In case the passive safety system is applied to the nuclear power plant, the safety of nuclear power plant may be increased relatively further than the active safety system is applied, but it needs to secure a relatively broader EAB. The expansion of EAB may result in a significantly increased cost of constructing the nuclear power plant.
Accordingly, an increasing need exists for a facility for reducing radioactive materials, which allows for application of a passive safety system to enhance the safety of nuclear power plant by resolving the problem of an expanding EAB.